Challenge Time
by WaiiKitsune
Summary: On what was supposed to be a 'normal' day in the "Assassination Classroom", Class 3-E found themselves a visitor in the form of a small, wide-eyed brunet challenging their homeroom teacher to a fight.


**Title:** Challenge Time

 **Summary:** On what was supposed to be a 'normal' day in the "Assassination Classroom", Class 3-E found themselves a visitor in the form of a small, wide-eyed brunet challenging their homeroom teacher to a fight.

 **Disclaimer:** Assassination Classroom belongs to Yuusei Matsui; Kateyo Hitman Reborn! belongs to Akira Amano

 **Pairings:** None

 **Warning:** Possible _**spoiler**_ alert (if you're not caught up to at least _Chapter 134, "Past Time" Arc_, of the manga). Possible OoC-ness

 **Note:** [For AC] Set somewhere in the 2nd Semester, before Itona joins; [For KHR] Set post-canon; Tsuna is in his 2nd year as Vongola Decimo (officially)

 **For Lightning515.**

 **Beta'd by mangaaddict26**

* * *

 **Challenge Time**

* * *

It was a normal day for Class 3-E of the Kunugigaoka Junior High School―or so they had been told. There wasn't supposed to be any special classes that day; it was a day of 'normal' classes and 'normal' curriculum. Yet, before homeroom had even finished that morning, the door of Class 3-E slid open, and a small, wide-eyed brunet peered into the room.

"This is… Class 3-E of the Kunugigaoka Junior High, right?" He asked, eyes sweeping through the classroom slowly.

"Certainly!" Koro-sensei replied cheerfully. "Are you a new transfer student?" He asked. Glancing over the brunet at Karasuma and Irina, who had came out of the staff room at the commotion, he added, "I wasn't aware we're getting a new transfer student…"

"There isn't any new transfer student." Karasuma stated promptly. He glanced down at the brunet before him, his mindset as an agent already kicking in to analyse the seemingly harmless kid before him. (And he knew Irina was doing the same beside him too.)

"Oh, no. I'm not a transfer student." The brunet chuckled nervously. "I'm, uhh… here to challenge you?"

"Challenge… You mean you're here to assassinate Koro-sensei too?" Nagisa questioned.

The students glanced at each other, then at the brunet, their minds simultaneously pondering the same question: _'What kind of assassin politely asks to kill their target?! (Even we're not that stupid!)'_

The mysterious brunet, on the other hand, blinked at them, then broke out into nervous laughter. "Ahh, no. No, I'm not part of the assassination attempt." He explained. "I'm here to request a friendly spar with Koro-sensei…?"

Silence fell over the classroom once more. _'…Ask for a friendly spar, he says. He just asked the perverted octopus who destroyed seventy percent of the moon to a 'friendly' spar. Who is this kid?!'_

Even Koro-sensei had looked stunned at the request, but he quickly snapped out of it. "Well, I'm always happy to provide a good learning experience!" Koro-sensei grinned cheerfully. "However, as a teacher, I have to give priority to my students, so the spar would have to be at a later hour."

"Of course! I… arrived a little too early, actually. I was planning to come around noon, but I had to, umm, get away from, uhh… traffic." The brunet smiled sheepishly. "It's alright. I understand your concerns. I can wait until you're free." Glancing at the room again, the brunet paused at the empty seats near the back of the classroom. "Is it okay for me to use that table there? I can work while I wait."

"Just wait a minute here. Aren't you getting a little too ahead of yourself?" Karasuma cut in with a scowl, glaring down at the brunet who, much to the agent's annoyance, is staring back at him curiously without a shred of fear. "Information about him is supposed to be classified and tightly monitored. Who are you and how did you know about that octopus?"

"Oh, that's right. I forgot to introduce myself." The brunet laughed. "My name is Tsuna. As for how I found out…" The quiet, nervous aura around the brunet disappeared almost instantly, and smile on that spread on Tsuna's face was mischievous, almost smug. "Well, I have my ways. In fact," His smile widened. " _I'm_ the one who should be surprised you don't know who _I_ am, Karasuma-san. Of course, once you figure out who I am, you should be able to figure out how I know."

The tension between Karasuma and Tsuna alone was making the class nervous, while simultaneously intriguing the two teachers. It wasn't often that Karasuma was left speechless; as the handler, the agent was required to know everything single variable involving Koro-sensei, yet a kid had popped out of nowhere with information he shouldn't know, demanding something that was almost ridiculous by normal standards, and to top it all off, he had all but taunted Karasuma boldly.

The 3-E students didn't know if the brunet was just that courageous, or plain suicidal, but nevertheless, they couldn't help but feel slightly impressed.

"Well!" Tsuna chirped. "Some quick answers to questions I'm sure you all have. Tsuna is not my full name, but it _is_ my real name, and most of my friends call me that. Again, I'm not here to assassinate Koro-sensei; I am neither an assassin or a hitman. Also, even if I don't look like it, I am nineteen, excuse my lack of growth spurt. As for why I'm here to challenge Koro-sensei… well, my tutor sent me here." Tsuna rattled off. "If you're wondering why I'm not telling you my full name, it's because if I told you that, I'll more than likely lose my trump card against Koro-sensei. If you're that interested, I'll be happy to tell you after I've fought him." The brunet concluded. "That should be all. Anything else?"

"If I may ask… who is your tutor?" Koro-sensei questioned.

"I'll have to save that for later too. If I told you the name, I'll also lose my trump card." Tsuna smiled. "I think I've caused quite a bit of disturbance, so I'll let you get on with your class. That _is_ alright, right, Karasuma-san?"

"…Do as you like." Karasuma snapped and left for the staff room.

Irina glanced at Tsuna one last time before turning to leave as well.

Tsuna stared down the hallway for a moment before quickly taking a seat at the back of the classroom. A call for attention came from Koro-sensei, and the class snapped back to class-mode, as the students forced themselves to push aside their burning curiosity for now.

* * *

 _'If the data is correct, I'm to fight an enhanced human whose fastest speed is Mach 20… if I go with hard flames right from the start… I can hit him—that is, if I don't go off-course...'_ Tsuna sighed as he pulled out the stack of Manila folders he had brought with him. _'At least I get to get away from the estate for now. Still, they're… probably panicking by now. Maybe I should send Shoichi a message in a bit…?'_

Shrugging to himself, Tsuna flipped open the first file he had and began reading. Having anticipated needing to wait (he _was_ , after all, barging into a classroom), the brunet had brought along a couple of mission files that needed to be approved and assigned. It didn't take very long, however, for Tsuna's attention to turn from the files to the ongoing class in front of him.

Despite being dubbed an 'Assassination Classroom', class was relatively normal. While Koro-sensei taught on the blackboard, any questions the students had were asked with an accompanying gunshot. The shot was fired as the students asked their questions, while Koro-sensei answered as he side-stepped the pink bullets without a glance. More often than not, Koro-sensei would be the ones answering the questions, and his face (or rather, the colour of his face) would change according to the answers given. All in all, Koro-sensei was an excellent teacher (despite his origins), and Tsuna could tell that whether they showed it or not, the students did, in fact, adore him.

' _Lucky them. They get a colour-changing face and occasional mockery. I get bullets and electrocution and bombs blown up in my face.'_ Tsuna sighed, shuddering involuntarily at the memories. _'Speaking of which, I wonder if it's too late to send Reborn on another mission… Nah. He knows I'm doing it to avoid him, and I'll die even more terribly now that he's… seven, the last time I checked.'_ Sensing yet another curious glance shot at him, Tsuna quickly turned back to his folders, as he had done every time his intuition warned him so. _'All that aside, it seems like Koro-sensei is starting to remember me, but hopefully teaching a class can distract him for a little while more… Hopefully.'_

* * *

"Today, instead of the usual drills, we will observe the fight between those two."

"… _Haaaaah?!_ "

Tsuna stared at Karasuma, resisting the urge to glare at the man he knew was hiding a smug smirk behind that calm face. Class 3-E, all clad in their sports uniform, looked at their physical education teacher, then to Tsuna, then at each other as they were rendered silent again.

Physical education was important to Class 3-E; it was where they gained their tools for assassination, after all. The 'Takaoka-Incident' (as the class had dubbed it) made them appreciate Karasuma much, much more, while their time on the island resort served to make them yearn for more training to make them better. While they were well-aware that they were merely students who only recently started training, and that there are many, many more capable assassins out there, Karasuma usually warned them of new assassins and they were always able to prepare themselves beforehand.

Tsuna's appearance was completely out of the blue. Moreover, if the brunet hadn't told them his age, they would have thought that Tsuna was about their age. And although Tsuna had said he had come on his mysterious tutor's instructions, contrary to his initial stuttering, the brunet had been confident when he challenged Koro-sensei. They couldn't accurately describe their feelings, but they knew that they were feeling what they felt with Itona: frustration.

It was ever bloodthirsty Karma who broke the silence with a wide grin, "Sure, why not? We're all curious to see the strength of the kid who challenged a moon-destroying octopus to a fight."

"Maybe we can learn a thing or two from Tsuna-san too." Nagisa agreed.

Tsuna stared at them in disbelief, before he gave a deep sigh. "You finally remembered who I am, haven't you, Karasuma-san?" The look in the agent's eyes said everything. "You are one vindictive man."

"Oh? And here I am, trying to help you." Karasuma replied. "Physical education is taken by me, which means that octopus has free time right now. It shouldn't take him too long to remember you." The smirk on Karasuma's face reminded him of his tutor, and Tsuna barely pushed down a shudder. "I'm more than happy to carry on with class if you want to lose your advantage."

"You know that it's unlikely that your students can learn anything from this fight, right?" Tsuna asked, voice soft enough for only Karasuma to hear. "I can fight him elsewhere while you have class."

"Then they'll be distracted and I need them focused." Karasuma answered.

"…Alright." Tsuna nodded. "Koro-sensei," He called in the direction of the school building. "I know you heard everything."

"Yes!" Koro-sensei chirped, appearing instantly by Karasuma's side. Giggling with a wide grin, he continued, "I'm ready to take you on anytime, Tsuna-kun."

And in that moment, Tsuna knew Koro-sensei had already remembered who he was. _'Well. I've faced someone who might be potentially worse… And I still have the first strike, because he wouldn't attack me first.'_

Tsuna turned towards the open field without another word, and Koro-sensei followed while Karasuma herded the class further back for safety. Deciding it was already too late for deceptions, Tsuna pulled out his Ring of the Sky out of his pocket and slipped it on, following it with his mittens. It was minute, but his Hyper Intuition enhanced vision caught Koro-sensei stiffening slightly at the ring.

' _This is probably the only time I can get a decent hit on him, so here goes…'_

Tsuna shifted into his Hyper Dying Will mode smoothly, hard flames simultaneously flaring from his gloves as he shot forward in the blink of an eye. He shifted quickly in mid-flight, driving his fist into Koro-sensei's face and sending the teacher flying. Tsuna shifted again, this time bringing his hands in front of him to slow himself down and he landed on the ground just as Koro-sensei flickered back in front of him.

"That was a wonderful punch, Tsuna-kun!" Koro-sensei smiled, spreading his 'arms' wide open. "I have certainly heard your resolve loud and clear."

With that, Koro-sensei flew forward, limiting his speed to just slightly faster than Tsuna, as he launched several tentacles at the brunet in several directions. Tsuna's eyes, now the colour of warm amber, narrowed as his flames flared from his gloves once more. The tentacles were much faster than Tsuna had anticipated; he had only managed to deflect the first few, and the rest had sent him flying. Tsuna jerked in mid-air, braking and accelerating towards Koro-sensei in the same moment, starting another round of attacks.

It was obvious to the audience that Tsuna couldn't match with Koro-sensei's speed. And, having been attempting to assassinate their homeroom teacher for months now, Class 3-E was well aware of how fast Koro-sensei actually was, and his current speed now was nowhere near that. They all knew that there was no chance of Tsuna beating Koro-sensei at all, but the brunet never seemed to care; charging towards his opponent relentlessly.

"It's not entirely hopeless." Karma spoke up suddenly, startling the class. Golden eyes gleamed under the sunlight, as a wide, almost bloodthirsty grin spread across his face. "The kid's improving pretty fast."

"Really? Tsuna-san just looks like he's getting beat by Koro-sensei." Sugino frowned.

"No, Karma is right." Karasuma injected, frowning faintly at the fight. "He starting to get a hang of reading that octopus' movements. He's getting hit less, and landing more hits in return. It's a slow process, but his pace of learning already exceeds the average, especially since this is the octopus we're talking about."

"Doesn't that hurt though?" Nagisa remarked thoughtfully. At the curious looks coming from his classmates, he clarified, "The fire burning on his hands on forehead. It doesn't look conventional, but that's definitely fire..." The looks changed from curiosity to confusion, and Nagisa blinked, "Wait, you can't see the fire?"

"I can." Karma said.

"Same here." Kayano added.

"I thought it was my eyes playing tricks on me. It looks really faint to me." Isogai laughed nervously, and Hayami echoed her agreement.

The rest of the class, however, merely stared at the five of them like they'd grown an additional head.

"They're not hallucinating." Karasuma said, glancing at the five students. "That is a special combat form passed down in his Family. The flames are special; it looks like a normal fire, and can burn like one too, depending on its density. The one he's using now is not very dense at all, and doesn't burn him." He explained. "All that aside, it is said that the flames he uses cannot be seen by just about anyone. The condition for seeing the flame, however, seems to be closely guarded by his Family; there are many rumours, but no accurate answer."

Out of the corner of his eyes, Karasuma saw Irina walking over from the school building to join the audience.

"Bitch-sensei," Kurahashi greeted cheerfully. "Can you see the flames Karasuma-sensei was talking about too?"

"I can." Irina nodded slowly, staring quietly at the fight. A couple of moments later, the hitwoman turned to Karasuma beside her and spoke, "Why is that kid here anyway?"

Karasuma shrugged. "Their entire Family is known to be unpredictable and insane." He remarked flatly. "His generation in particular."

"No, why is _he_ here, Karasuma." Irina clarified. Turning to the agent, she continued in a low voice, "That man should have been the first person the higher-ups tried to contact when this whole thing happened, right? The fact that he's still standing here means they were rejected, right?"

Irina had deliberately kept her voice soft enough that only Karasuma could hear, but the dark expression on her face had caught the attention of some of the more perceptive students in class. The agent sighed softly, gently rubbing the bridge of his nose as he thought back to the situation when the Moon was first obliterated.

The world had been in a panic, and the world leaders were desperately trying to get the situation under control. When Koro-sensei appeared and claimed responsibility for the Moon's destruction, threatening to destroy the Earth as well, the world leaders had agreed to Koro-sensei's request to teach Class 3-E at Kunugigaoka Junior High for the sake of using that time to assassinate him. With the Earth's survival at stake, the world leaders had immediately reached out to a certain hitman who held the reputation of being the 'World's Greatest Hitman'. Unfortunately, while they managed to hunt down the somewhat elusive hitman, the world leaders had been rejected despite their large monetary award.

"Rejecting the job doesn't necessarily mean he was ordered to." Karasuma answered calmly. "The hitman has the right to refuse any jobs that he doesn't want."

"This job is every underworld killer's dream. It's practically a done deal for someone of _his_ calibre. There is no reason for him to turn down the job unless his boss had forbidden him." Irina argued.

"Whatever the case is, he rejected the job offer. This means that everyone else has a shot at getting that award—including _you_ , Irina." Karasuma retorted.

"Umm… Sensei…?"

The two teachers broke eye contact and turned to see a number of students looking curiously at them while the rest kept their eyes on the fight in the distance.

"Just who is Tsuna-kun, sensei?" Yada questioned. Having heard her, most of the remaining students turned their attention to the teachers as well. "We know he said he wasn't an assassin, but how he fights is definitely not normal. In fact, it looks like something only an assassin would do…"

"He is…" Karasuma trailed off. _'There are too many things I cannot tell the student about him. Even the things I_ can _say have to be covered up as well.'_

"He was telling the truth; that kid is not an assassin or a hitman. He has ties to hitmen like Lovro-sensei, but he is not one himself." Irina answered instead. "Don't let his appearance fool you; he may look very young and innocent, but he is one of the most powerful men in the world." She continued, crossing her arms. "I don't mean it in the physical strength, of course; there are many people who can beat him in a physical fight pretty easily. That kid is very, _very_ influential. One word from him can change a situation very drastically."

"And he's only nineteen…?!" Sugino gasped.

"His influence is inherited, but he himselfhas the ability to back it up." Irina said. "In fact, his natural charisma added to his influence. People tend to respect him and fear him at the same time."

"They have nothing to fear if they do not do anything unforgivable." Koro-sensei's voice came, disturbingly close, and the audience turned to see the octopus blurring beside them. "Tsuna-kun has a reputation of being very kind for someone of his profession."

"Unforgivable… like what?" Nagisa asked curiously.

"Like betraying your friends and family; back-stabbing them, in particular." Tsuna's voice came next, and the audience turned once more to see the brunet appearing next to Koro-sensei, who was no longer flickering in place. "That and human experimentations are on top of the list."

"Tsuna-kun—" Koro-sensei started, voice tight and low, but the brunet cut him off.

"—One of my family has been a victim of that, you see, and even now, years later, he's still rather… bitter about it." Tsuna stared at Koro-sensei calmly as he explained. _'Bitter is a bit of an understatement. I could probably add 'insane' to that, but I'm keen to believe my Guardians are mostly insane to begin with. That aside, mentioning the words 'human experimentation' around Mukuro is a sure way of starting a massacre.'_

"That's… horrible…" Okuda spoke up quietly, gaining the attention of everyone present instantly. "Science is important to progress and evolution, but to use people to do it… That's just not right…!"

The normally shy and quiet girl was close to tears by the end of her short speech, and Koro-sensei moved to comfort his student, only for him to be beaten by the brunet beside him. Tsuna gently placed a hand on Okuda's head and, with the same amount of gentleness, slowly patted her head comfortingly. Okuda froze momentarily, startled at the sudden comfort, but slowly lifted her head to see a genuinely kind smile on Tsuna's face.

"You're right. No matter the reason, there shouldn't be a need to experiment on humans, especially if the ones being experimented on were forced to." Tsuna said softly. "Don't worry, little miss. Illegal human experiments won't happen again as long as I'm around. I promise." Okuda sniffled and nodded. Tsuna smiled, retrieving a handkerchief from his pocket with his free hand and offered it to Okuda, who accepted it gratefully. "Now, don't worry and smile a little alright? Your teachers and classmates are getting a little worried." Okuda gasped softly and glanced at the semi-worried looks. She immediately tilted her head down, a faint blushing spread across her face as she muttered a barely-audible apology.

"Hey." Karma smiled dangerously at Tsuna. "Just who the hell are you?"

"K-Karma-kun…!"

The redhead ignored whoever called him warningly (he was pretty sure it was Nagisa anyway) and went on, "You been spouting a lot of stuff and so has Bitch-sensei, but you look like a kid just like us." Karma's smile only widened, and got more bloodthirsty. "If you're that powerful, then prove it. And without the fancy flame tricks this time."

"Fancy flame…" Tsuna's eyes widened. "You saw that?"

"Karma, Nagisa and Kayano all saw the flames clearly." Karasuma explained. "Isogai and Hayami caught faint glimpses of it."

' _They saw the Dying Will flames…? It's not easy to see the flames if you haven't encountered them in close quarters before, but they saw it at that distance…? Not to mention the Sky flames I used were soft flames; the purity is low and the flame is fainter than hard flames.'_ Tsuna frowned. _'Reborn told me before that one of the exceptions for seeing the Dying Will flame is when you put your life on the line. After all, Dying Will flames are more or less linked to one's life force. These kids… They're really—'_

"Tsuna-san?"

"…Oh? Ahh. Sorry." Tsuna blinked, snapping out of his thoughts. "Right. I mentioned telling you my name after the fight right? My name is Sawada Tsunayoshi." (He did not miss the teachers stiffening momentarily.) "I was born and raised in Namimori, Japan, but I currently live in Italy, where the organisation I am the leading is headquartered."

"You grew up in Japan but you're working all the way in Italy?" Sugino asked curiously.

"I inherited the organisation as its tenth head. Normally, the position would be inherited by the previous head's direct descendants, but as Ninth's sons met with unfortunate ends, I am the last legitimate successor to the organisation. In fact, in terms of legitimacy, I would have been the first in line for inheritance, since my ancestor was the one who founded the organisation and I am his only direct descendant." Tsuna explained.

His eyes swept across his audience, and a faint smile lifted the side of his lips when his Hyper Intuition notified him of one student in particular already putting together the pieces of information he and the teachers had given over the day.

"And that's all I can say. I've given you enough information for your classmate to find out the rest." Tsuna continued, swapping his grin for a firm stare. "I _will_ warn you first. Your teachers were not exaggerating. I _do_ have a lot of influence, and knowing too much about me could be dangerous. I sincerely suggest consulting with Koro-sensei, Karasuma-san and Irina-san before attempting to quench your curiosities."

Just then, as if to emphasise that the conversation is over, Karasuma checked his watch and declared that physical education class was over.

* * *

At Koro-sensei and Karasuma's request, the 3-E students shuffled back into the building, conversation flying furiously between them. It didn't take long for their heads to pop out of the classroom window, still watching their guest and teachers curiously, but at least they were now far enough to be out of earshot.

"Well then," Tsuna turned away from the direction of the classroom. "Now that there's no need for filters…"

"Did Reborn really send you here, Tsunayoshi-kun?" Koro-sensei questioned without preamble.

Tsuna smiled mysteriously and asked instead, "How does your body feel, Koro-sensei?"

While Karasuma and Irina were both alarmed, turning to Koro-sensei to visually scan for injuries, Koro-sensei merely stared back at Tsuna, meeting the brunet's almost mischievous looking eyes.

Finally, he sighed, "Tsunayoshi-kun… How much do you know?"

"Koro-sensei…" Tsuna smiled. "I am Vongola Decimo. Tenth head of one of the largest, most powerful and bloodiest Mafia Family in Italy. I'm sure you know the difference between us and the government." He paused, glancing at Karasuma out of the corner of his eyes. "The government is bound by the law, and the duty to protect the masses. We're not. Not to mention…" Tsuna's smile changed from a smug one to one of pride. "My subordinates are very, _very_ talented at their respective area of expertise."

"You know _everything_." Koro-sensei concluded.

"If you know everything," Irina spoke up. "Why did you not join in the assassination efforts? Why did you order your strongest hitman to stay away from this?"

' _Judging from their faces and my intuition… Karasuma-san and Irina-san don't know the entire truth behind Koro-sensei. And although I'm the one who dug into it, the truth is not for me to tell.'_ Tsuna frowned. "I admit I ordered Reborn to stay away from this assassination should he be requested, but Reborn himself didn't want to accept it to begin with. I believe he mentioned something about not wanting to kill one of the few people in this world who can give him a challenge." He glanced at Koro-sensei as he answered. "As for why I stopped him… It's simple. I don't believe in senseless killing."

"Senseless?" Irina echoed in disbelief. "We're talking about the man who destroyed seventy percent of the Moon and is threatening to blow up the Earth next March!"

"So he did." Tsuna nodded in agreement. "But right now, he's here, in Kunugigaoka Junior High, teaching a class believed by the masses to be useless and talentless. Right now, in my eyes, though I see the world-destroying enhanced being, I also see a teacher who truly and sincerely cares for his students." Tsuna replied calmly. "Just because someone is dangerous, it doesn't mean that they deserve to be killed, Irina-san.."

"I understand that…!" Irina retorted. "But…"

"…If ever the day comes where Reborn starts to hate the world and wants to destroy it, he can surely do it without breaking a sweat. I think we can agree that he's as least as dangerous as Koro-sensei, if not more. But this man… This powerful, dangerous man… he is my tutor." Tsuna said softly. "If it wasn't for Reborn, I wouldn't be who I am today. He didn't just teach me academics; he also taught me how to live." The brunet glanced back at the school building, where the students were still watching them from, as he continued, "I know what they feel. I know what it's like when the whole world looks down on you. When I think back on it, Reborn was probably the first person who believed in me, even if he repeatedly questioned himself and probably regretted that belief almost all the time."

"You won't kill him or allow your hitman to kill him, but yet, you don't stop the Assassination Classroom. Quite an irony, don't you think?" Karasuma spoke up, and Tsuna turned to him questioningly. "You see yourself in the students, and your tutor in that octopus. Are you telling me you would kill your tutor if the same scenario happened with your tutor instead?"

"…A few years ago… I was fighting to save Reborn. The upcoming battle was one that was far more dangerous than anything I've encountered and, in fact, the chances of me winning was close to zero. And if I had lost… I would die. Reborn tried to stop me even though me not fighting would possibly result in _his_ death instead. Back then, Reborn told me that as a hitman, he never expected to die a decent death. For the first time, I was truly _**angry**_ at Reborn. That my tutor, who had unfairly pushed so many things on me, had been thinking such sad thoughts throughout our time together. I told him as much too—that someone who has been thinking of such sad thoughts was an utter failure as a tutor." Tsuna spoke. "Karasuma-san, I would do _anything_ to save Reborn. But if, after exhausting every possible method, the only way to save him is to kill him… Then I will kill him with my own two hands."

* * *

"What do you think they're talking about?" Kayano asked. "It's rare to see Koro-sensei looking so serious…"

"Karasuma-sensei's eyebrows look even more furrowed today." Nagisa nodded. He turned to the male beside him. "Karma…" The redhead hummed in acknowledgement, but did not turn away from the group in the distance. "Do you know what they're talking about?"

"No." Karma grumbled. "That kid is deliberately back-facing us and the rest are standing at an angle. At that angle, not even Ritsu can read what they're saying."

Nagisa sighed softly as he turned back to the classroom.

They had all skipped changing back into their uniforms in favour of observing the group outside, but they were standing far enough that it was a fruitless effort. The class split up then; some deciding to keep watching from the windows, some deciding to change out of their sports uniform, while others lounged around like they were on lunch break.

"Terasaka-kun…?" Nagisa called curiously. "What… are you doing?"

The class delinquent had his trademark deviously sneer on his face as he stood in front of the desk Tsuna had been using earlier. By now, most of the class' attention had been drawn by Nagisa's call, and they were looking between Terasaka and the folders still sitting on the desk.

"What does it look like? I'm going to find out just who that annoying brat is." Terasaka declared. "He said he brought work to do and this was what he was doing, so there has to be something about him in here."

"Ohh?" Karma hummed teasingly. "Looks like you _do_ have a brain in there, Terasaka."

"Shut up, Karma." Terasaka snapped automatically, looking at the folders in front of him greedily. "Let's see who that brat is…" He muttered, reaching a hand towards the top folder.

Right before the tip of his fingers could even touch the folder, however, the door of the classroom slid open with a loud bang, and Terasaka's hands froze in mid-air.

The air in the classroom changed instantaneously as every single student of the assassination classroom went on guard, for the man who had all but yanked the classroom door open was someone none of them had seen before. And given that there was only one way into the mountain campus, the man before them must have had slipped past Koro-sensei _and_ Karasuma effortlessly.

But even without knowing that fact, there was no mistaking what the aura around the man was telling them.

' _This man is dangerous. He's stronger than Karasuma-sensei… and possibly even Koro-sensei.'_

The students couldn't speak—none of them dared to. Even the best in combat in the class was taken aback by the sheer presence of the man, and every single one of them simply froze in place. Even Terasaka's hand was still frozen above the folders, unable move his hand away.

The man—a tall, slim man in a neatly-pressed suit, with a orange-banded fedora atop silky black locks—glanced around the classroom, seemingly taking in the appearance and body language of each and every student present.

"Chaos." The man spoke in a deep baritone, lifting a hand to tilt his fedora in greeting. The single word broke the frozen air of the classroom, and the students dazed minds snapped back into attention. "I'll be taking that." The man continued, glancing at the folders on Tsuna's desk. Without waiting for a response, the man gathered the folders, occasionally flicking through them, before putting them all into the bag that Tsuna had brought with him. " _Ciao_." All done, the man left, bag in hand and disappeared behind the classroom doors.

"Who…" Sugino breathed. "Who was that…?"

"I don't know…" Nagisa mumbled. "But I… I couldn't even move."

"None of us could." Kayano countered.

"That old man is insanely strong. Even if all of us worked together flawlessly, we still wouldn't be able to land a hit on him." Karma ground out. "He's probably an assassin, but he's not here to kill Koro-sensei either."

"Maybe he's with Tsuna-san." Nagisa remarked.

"Probably."

* * *

Reborn was going to take extreme delight in _murdering_ his Idiot Student No. 2, aka Vongola Decimo.

Reborn pulled out an empty Box and sealed Tsuna's bag in, before tucking the Box back into his suit pocket. Leon poked out from his pocket, knowing his owner was now out of sight of the children, and climbed its way up to Reborn's shoulders with a soft purr as the hitman strode out of the class building and towards the group in a distance.

It took no effort for Reborn to keep his footsteps silent, but the famed Vongola Hyper Intuition wasn't famed for nothing, and the hitman saw his student straightening and tensing abruptly in the distance.

"Don't even _think_ about it, Dame-Tsuna." Reborn called out.

The three teachers turned to look at Reborn in surprise, but Tsuna didn't dare to turn around.

The hitman continued stalking towards the group, no longer hiding the sound of his footsteps as he revelled in the sight of his student twitching nervously at every step. Reborn raised a hand, and Leon crawled onto it automatically. A rainbow-coloured glow later, a Leon-green coloured One-Ton mallet appeared in Reborn's hand, and said mallet was mercilessly slammed onto his student's head without warning.

Tsuna clutched his head, biting down a pained scream as a tear was squeezed from his eyes. "Are you trying to kill me, Reborn?"

"Well, you're still alive so obviously I wasn't trying hard enough." Reborn snarked. Turning to the somewhat-flabbergasted teachers, the hitman tilted his fedora as he greeted, "Chaos. Apologies about my Idiot Student No. 2 here." Obsidian orbs gleamed from under the brim of the fedora as Reborn continued, "I believe they call you _'Koro-sensei'_ now. It's been a while."

"Yes… It's been a while, Reborn." Koro-sensei answered. "I see you've grown."

"He's not supposed to." Tsuna huffed.

"Well if someone didn't disappear from the estate without a word and without escorts, I wouldn't have needed to, would I?" Reborn retorted.

"How did you even get here so fast?" Tsuna demanded. "I calculated!"

"My mark was stupider than expected. I finished early." Reborn answered. "Gokudera was quick to spill the beans when I called the estate. Sadly, he hasn't learned the lesson that you know better than to leave without your full set of equipment, and locating you takes all of five minutes to call Spanner to locate the tracker in your headphones." The hitman smirked at Tsuna's increasingly pale face. "And yes, I'll be tripling your training."

The Vongola Decimo opened his mouth to protest, but the look on his tutor's face made him shut it rather obediently.

"I'm guessing you didn't send your student then, Reborn." Koro-sensei remarked.

"No." Reborn replied, glancing sideways at Tsuna (who had the cheek to look _sheepish_ ). "In fact, I specifically remembered warning him away from here."

"You warned me, but didn't specifically forbid it." Tsuna chirped. With a wide grin, he continued, "An advisor should always be clear and concise with his words, leaving no room for the advised to find loopholes in."

Reborn whacked him over the head with the One-Ton Mallet without any remorse.

As Tsuna whimpered pitifully while clutching his head, the hitman glanced at the two quiet teachers.

Karasuma had been observing Reborn ever since he arrived, quietly analysing the hitman's strengths and weaknesses (not that Reborn was giving him much to work with). The agent had also been watching the interaction between his student and himself, as if wondering how their relationship worked. Irina, on the other hand, had been staring at Reborn with wonder in her eyes. As the 'World's Strongest Hitman', Reborn is as much of a 'celebrity' in the world of hitmen and assassins as Tsuna was in the Mafia. And for a hitwoman like Irina, meeting Reborn was likely a once-in-a-lifetime event for her. (To be fair, depending on what his idiot student decides, it just might be.)

"Tsuna," Reborn called after a minute. "Are you done here?"

All traces of his previous whining disappeared instantly as the Vongola Decimo smiled calmly, "I just need to check one thing…"

Reborn nodded, and in the next instant, two of Koro-sensei's tentacles dropped to the ground with accompanying gunshots, while part of his head melted to the ground in gooey yellow blobs when the man had failed to dodge in time. Karasuma and Irina froze, while calls of _"Koro-sensei!"_ rang out in the distance. Reborn slowly tucked the still smoking gun away while Koro-sensei regenerated the damaged parts.

"Well. That was close." Koro-sensei announced cheerfully.

Reborn ignored him and questioned instead, "Are you done here?"

"Yep." Tsuna nodded this time. "Sorry for taking up so much of your time, Koro-sensei. This has been a very insightful visit." With a sincere smile, Tsuna bowed politely. "We'll be taking our leave now." They barely took a dozen steps when Tsuna stopped abruptly and spoke, "One last thing. Koro-sensei… I look forward to hiring you one day."

Tsuna and Reborn strode out of Kunugigaoka Junior High's mountain campus just as the students came running up from the classroom.

* * *

"Reborn." Tsuna called once the pair was completely hidden in the forest path. "Revert."

One sunny yellow glow later, and the hitman shrunk to Tsuna's chest height with a huff.

"I know it's my fault, and your original body makes it easier to move around, but I still have to say it. Just because you have the Sun flames of Activation, you shouldn't abuse it. Forcing your body to grow into your original age using Sun flames can damage your body; especially since you're all still stuck in the sporadic growth stage." Tsuna frowned.

"That perverted illusionist wasn't that far off when he called you _'motherhen'_." Reborn muttered under his breath. Tsuna spluttered incoherently, but the hitman simply ignored it and continued, "So, are you going to explain yourself? Why are you so interested in Cel?"

"Reborn, I can count the number of people you actually respect with _one_ hand. And Koro-sen— I mean, Cel, is known as the World's Greatest Assassin. Why _wouldn't_ I be interested." Tsuna deadpanned. Reborn stared at his student, unimpressed. "That aside, one of the few things I didn't need to dig too deep to find out was that Cel was currently teaching the exiled class of Kunugigaoka. I was curious about how it turned out. Three hours in that classroom, and I concluded, unsurprisingly, that Cel was an excellent teacher. Kinda like you, but with less explosions and more mockery and taunting."

"A simple curiosity like that wouldn't have been enough for you to sneak out of the estate and travel all the way to Japan to observe him." Reborn stated. "So, what did Verde find out?"

"…Everything." Tsuna replied. "It took a while, but Spanner managed to hack into the files on the experiment for me. I passed those files to Verde to translate into English, while I hunted around for information on Cel himself. What happened to him… wasn't pleasant. Someone close to Cel betrayed him, in one of the worse ways possible." The brunet's eyes darkened for a long moment before he continued, "Verde briefly explained to me what the experiment was about, and I have him digging further, but… Whatever they did, it's biochemical."

"And if there's anyone who can undo it, it's the Da Vinci's Reincarnation, huh." Tsuna nodded. Reborn glanced over at his student as his eyes narrowed. "…Deliberately sneaking out a day before I'm expected to return from my mission and coming out here alone was also part of your plan, wasn't it." He stated. Tsuna's lips twitched into a small smile. "You knew that when I found out, I'd come here alone without informing your Guardians where you were. Not only that, you were also counting on me finishing my mission early."

"Before I left, I instructed Spanner to tell anyone, if they asked, that I forgot my headphones when I snuck out. He was allowed to tell one person alone where I was―you." Tsuna confirmed. "What I did and will do in the future, I will not involve Vongola in this. This is my personal agenda, and I'm prepared to inform Vindice as much when they finally decide to show up. That's why I made Spanner keep silent; the only ones in the Family who knows are the three of us." He paused briefly, allowing Reborn to take in what he said. "His regeneration speed slowed down earlier. Barely noticeable, but it was slowed."

"Harmonization factor?" Reborn quirked an eyebrow.

"Whatever they did to Cel, it resulted in what we'd call an uncontrollable outbreak of Sun flames, represented by his near-instant regeneration. Normally, the Sky flames' Harmonization factor will break it down, but the combination of the speed of regeneration and the low purity of the Sky flames I used helped to stabilize it instead." Tsuna explained. "The tranquillity factor of Rain flames will work even better, but whether it's Sky or Rain, it's only a temporary measure; his body is far too unstable."

"Are you assuming he can be saved?" The hitman questioned.

"…I really hope he can be." Tsuna answered quietly after a long moment. "But when the deadline comes and he can't, then… I will send you to kill him, Reborn. I think… you're the only one qualified to kill him."

"…Understood, Decimo."

* * *

 _To: Karasauma Tadaomi_

 _From: [Unknown Sender]_

 _Subject: Assassination_

 _I will give the students a new deadline. On March 12, should the current situation persist, I will send the World's Greatest Hitman to assassinate Koro-sensei, free-of-charge. Do pass that message along to your higher-ups as well._

 _[End]_

* * *

Notes ( _ **spoilers**_ alert):

1\. Cel is the Italian God of Death and the Underworld. It's Tsuna and Reborn's nickname for Koro-sensei.

2\. I'm making a distinction between assassins and hitmen here (technically, they're different to begin with). So Reborn is the World's Greatest _Hitman_ , Koro-sensei is the World's Greatest _Assassin_.

3\. At this point where the fic is set, Tsuna doesn't know about the other research team the world leaders has. So, Tsuna doesn't know for sure if Koro-sensei can be saved, but he believes that if anyone can, it's Verde.

4\. According to Reborn wiki, the Dying Will Flame are more akin to a real flame, and can therefore be seen easily. I messed up and read that as "it can't be seen by everyone", and had to come up with a criteria for certain individuals seeing Tsuna's DWF. I asked my two betas about the theory, and they said it's plausible, so I ran with it.

5\. Tsuna had three reasons for keeping his actions between himself, Reborn and Spanner (plus Verde). First, Reborn was the one who told Tsuna about Koro-sensei to begin with (plus he'd never be able to hide anything from Reborn) and he needed Spanner to collaborate with him (Tsuna trusted Spanner to keep his secret, plus few people will suspect Spanner). Second, Tsuna had looked into Koro-sensei out of his own curiosity, and he decided to pursue it on his own accord; he didn't want to drag the Vongola Famiglia into his personal agenda. Lastly, this authoress is absolutely terrible with crowding, so if any more people were involved, we would have a lot more silent audiences (which you've already seen with over half of 3-E in this fic).

6\. Minor thing, but the last text is written in my limited knowledge of Japanese texting (which is pseudo-email form). Please forgive any inaccuracies.

* * *

A/N: My first AC plus crossover fic! Most of the ACxKHR fics I've seen were all of present!10thGen somehow joining 3-E as a transfer student and I wanted to try something else. To be honest, this started as a fight-scene practice, but it's incredibly hard to write Koro-sensei fighting Tsuna with his tentacles (because even typing this _sentence_ sounds super wrong; can you imagine several _paragraphs_ of this?), so it turned into an actual plot bunny.

There were two parts to the plot bunny. The first is a question I have with regards to any ACxKHR crossovers: with the involvement of the KHR-verse, you'd think Reborn can take care of Koro-sensei, no sweat. The second is the parallel I drew between Koro-sensei and Reborn, and Tsuna (plus 10Gen) and Class 3-E. Apart from their methods of teaching, Koro-sensei's circumstances feels somewhat similar to Reborn to me. (Koro-sensei technically forced into the experimentation, while Reborn was cursed against his will.) Given what happened in the Arcobaleno Representative Battle arc, it's not much of a stretch for Tsuna to want to save Koro-sensei, despite the fact that he had never even met Koro-sensei. (Plus, even without the parallels, I'm keen to believe Tsuna is good enough to want to save Koro-sensei anyway.)

Because it's a crossover with KHR, naturally I had to bring in the Dying Will Flames and their characteristic. In the case of Koro-sensei's rapid regeneration, I connected it to the Sun flame's Activation factor, but in an uncontrollable state. As explained in the fic, although the Sky flames' Harmonization factor usually causes things to turn into stone, the rate of Koro-sensei's regeneration was too fast for the Harmonization factor to take effect, and the clash caused a Tranquility effect instead.

So there! This is the first time I'm writing in the Assassination Classroom fandom, and I apologize for the number of people who didn't get to speak. I hope I got all of them in-character though (with the exception of Tsuna, who is in my usual Vongola-Decimo style). I'm pretty happy at how this turned out (this was also waaaaaaay longer than expected). Hope you guys enjoyed this too! Ciao ciao~


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